Blends of Synthetic Distillate and Biodiesel for Low Nitrogen Oxide Emissions from Diesel Engines

ABSTRACT

This invention shows how to make and use a biodiesel-based fuel in diesel engines without incurring the NO x  penalty. Embodiments primarily relate to an optimum range of bulk modulus of compressibility for biodiesel blends, which results in generating “NO x  neutral” biodiesel blends or to formulate biodiesel blends with lower NOx emissions than conventional petroleum diesel fuel. These biodiesel blends preferably comprise synthetic paraffinic middle distillate derived from a hydrocarbon synthesis to generate synthetic environmentally-friendly diesel fuels.

CROSS-REFERENCE TO RELATED APPLICATIONS

This non-provisional application claims the benefit of U.S. ProvisionalApplication No. 60/551,574, filed Mar. 9, 2004, which is herebyincorporated by reference in its entirety

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with United States Government support underCooperative Agreement DE-FC26-01NT41098 awarded by the U.S. Departmentof Energy and entitled “Development & Evaluation of New Processes forthe Production of Ultra Clean Fuels from Natural Gas”. The United StatesGovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the blending of synthetic productsderived from synthesis gas and biomass for the production ofenvironmentally-friendly diesel fuels, which generate very low levels ofNOx emissions when used in compression ignition engines.

2. Background of the Invention

A number of performance specifications have been established for dieselfuels of different grades depending upon service application. A numberof different properties are set out in these specifications including,for example, flash point, cloud point, pour point, viscosity, sulfurcontent, distillation range, gravity and ignition quality. Of these, theignition quality is an important parameter and is usually expressed ascetane number (CN) determined by the standard ASTM test method D-613.Diesel fuel properties given the most attention are cetane number,aromatics content, and sulfur content.

Internal combustion engines produce emissions containing water vapor,carbon dioxide, carbon monoxide, unburned hydrocarbons, oxides ofnitrogen (NOx), carbonaceous soot and other particulate matter. Federaland state regulations dictate the amount of these and other pollutants,which may be emitted. Oxides of nitrogen, products of incompletecombustion, and particulates are considered atmospheric pollutants.

Sulfur is, of course, associated with the production of acidic oxides ofsulfur, an atmospheric pollutant. These compounds have been reported tocontribute to “acid rain.” During combustion of fuels that containsulfur compounds, oxides of sulfur (SO_(x)), such as sulfur dioxide(SO₂), and sulfur trioxide (SO₃) are produced as a result of oxidationof the sulfur.

Some fuels may contain nitrogen compounds that may contribute to theformation of oxides of nitrogen (NO_(x)). NO_(x) are primarily formed athigh temperatures by the reaction of nitrogen and oxygen from the airused for the reaction with the fuel.

Aromatics in diesel fuels are also considered undesirable, not only fortheir adverse effect on ignition quality, but also because they havebeen implicated in the production of significant amounts of particulatesin the engine exhaust.

Current environmental regulations are setting stricter specifications ondiesel fuels, especially to address sulfur, nitrogen, and particulateemissions. Federal and state legislative bodies and agencies have issueda number of rules applicable to the production of clean diesel fuel inattempts to reduce emissions. Many technologies have been developed forreduction of SO_(x) and NO_(x), but few can remove both types ofpollutants simultaneously in a dry process or reliably achieve costeffective levels of reduction.

A rapid series of diesel fuel improvements has been introduced in mostparts of the developed world to provide reductions in particulates andNO_(x) from the vehicle fleets in current operation as well as tofacilitate the introduction of after-treatment devices. Reducing thesulfur content and the “heavy end” of the fuel have been the keychanges. Reducing the sulfur content typically involves the reduction offuel sulfur via hydrotreating to levels as low as 10 parts-per-million(or ppm) as for example, in Swedish Mk 1 fuel. Other fuel parameterssuch as aromatics and cetane have also been the subject ofinvestigation.

Because of increasingly stringent federal and state regulations, demandfor clean diesel fuels for compression ignition engines that containvirtually no sulfur and nitrogen, and with lower aromatic content, willlikely increase. Clean diesel fuels having relatively high cetane numberare particularly valuable.

Clean distillates can be produced from petroleum-based distillatesthrough severe hydrotreating at great expense. Such severe hydrotreatingimparts relatively little improvement in cetane number and alsoadversely impacts the fuel's lubricity as discussed in an article byBooth & Wolveridge in Oil & Gas Journal, Aug. 16, 2003, p 71-16. Fuellubricity, required for the efficient operation of fuel delivery system,can be improved by the use of costly additive packages. Speciallymanufactured fuels and the incorporation of special fuel components suchas biodiesels and Fisher Tropsch diesels, have been gaining attention.

The production of clean, high cetane number distillates fromFischer-Tropsch synthesis has been discussed in the open literature.Generally, Fischer-Tropsch synthesis converts a mixture of hydrogen andcarbon monoxide (called syngas) to a multitude of hydrocarbon moleculesfrom 1 to 100 or more carbon atoms. Sources of synthesis gas can beobtained from reaction of methane or natural gas with an oxidant (waterand/or molecular oxygen) and/or from gasification of coal, petroleumcoke or biomass. The mixture of hydrocarbons from Fischer-Tropschsynthesis can be distilled, and its fractions submitted to varioushydroprocessing schemes to generate valuable products such as gasoline,diesel, wax, and/or lubricants. Since the Fischer-Tropsch synthesistends to produce primarily linear hydrocarbons (i.e., normal paraffins),the Fischer-Tropsch derived diesel product (FT diesel) is ofparticularly good quality. Fischer-Tropsch derived diesels typicallyhave high cetane number (greater than 70), and have very low sulfur,nitrogen and aromatic contents. Research from Clark and coworkersdisclosed in a 1999 article in Society of Automotive Engineers (SAE,Warrendale, Pa.) Technical Paper No. 1999-01-2251, and other studieshave shown that FT diesel fuels yield lower NO_(x) emissions. Eventhough the low aromatic content of FT diesel yields good thermalstability and reduced tendency to form deposits in engine; the absenceof aromatics can result however in swelling of elastomers in vehiclefuel systems, and hence seal leakage problems may arise. Moreover, thedisclosed hydroprocessing schemes for preparing Fischer-Tropsch deriveddistillates also leave the diesel lacking in one specific property,e.g., lubricity. The Fischer-Tropsch derived distillates would requireblending with other less desirable stocks or the use of costlyadditives, such as a lubricity-enhancing additive.

Renewable diesel fuels are fuels that are used in diesel engines inplace of or blended with petroleum diesel, but are made from renewableresources such as vegetable oils, animal fats, or other types ofbiomass, such as grasses and trees. Today Fischer-Tropsch diesel is madefrom fossil fuels (coal and natural gas), but a “biosyngas”, a synthesisgas generated from biomass, could be used to make clean liquid fuels inthe future.

Biodiesel is an example of a renewable diesel fuel that is used acrossthe world today. Biodiesel can be manufactured from vegetable oils,animal fats, waste vegetable oils (such as recycled restaurant greases,called yellow grease), microalgae oils, or any combination thereof,which are all renewable. These feedstocks can be transformed intobiodiesel using a variety of esterification or transesterificationtechnologies.

Biodiesel use is growing rapidly, increasing from about 7 milliongallons in 2000 to more than 20 million gallons in 2001, with additionalproduction capacity available to quickly accommodate further growth.Current U.S. biodiesel production is based largely on soybean oil andused cooking grease, both of which are abundant feedstocks. The mostfrequently used biodiesel feedstock in Europe is rapeseed (canola) oil.No matter what the process or the feedstock used, the produced biodieselmust meet rigorous specifications to be used as a fuel. Fuel-gradebiodiesel must be produced to strict industry specifications, as isdescribed in the American Society for Testing and Materials method, ASTMD-6751, in order to insure proper performance in diesel engines.Technically, biodiesel is defined as a fuel comprised of mono-alkylesters of long chain fatty acids derived from vegetable oils or animalfats, designated B100, and meeting the requirements of ASTM methodD-6751. Fatty-acid alkyl esters are actually long chains of carbonmolecules (8 to 22 carbons long) with an alcohol molecule attached toone end of the chain. Biodiesel refers to the pure fuel without blendingwith a diesel fuel derived from fossil fuels. The biomass-derived ethylor methyl esters can be blended with conventional diesel fuel or used asa neat fuel (100% biodiesel). Biodiesel blends are denoted as “BXX” with“XX” representing the percentage of biodiesel contained in the blend(i.e.: B20 is 20% biodiesel, 80% petroleum diesel; B100 is purebiodiesel). Pure biodiesel typically requires special treatment in coldweather, due to a high pour point. Biodiesel, as defined in ASTM D-6751,is registered with the U.S. Environmental Protection Agency (EPA) as afuel and a fuel additive under Section 211(b) of the Clean Air Act.Biodiesel is used, mostly as a 20% blend (B20) with petroleum diesel, infederal, state, and transit fleets, private truck companies, ferries,tourist boats, and launches, locomotives, power generators, home heatingfurnaces, and other equipment.

Biodiesel is non-toxic and biodegradable. It is safe to handle,transport, and store, and has a higher flash point than petroleumdiesel. Biodiesel can be stored in diesel tanks and pumped with regularequipment except in colder weather, where tank heaters or agitators maybe required. Biodiesel mixes readily with petroleum diesel at any blendlevel, making it a very flexible fuel additive.

One of the unique benefits of biodiesel is that it significantly reducesair pollutants that are associated with petroleum diesel exhaust. It canhelp reduce greenhouse gas emissions, as well as sulfur emissions sincebiodiesel contains only trace amounts of sulfur, typically less than thenew U.S. EPA rule finalized in 2001 that will require that sulfur levelsin diesel fuel be reduced from 500 ppm to 15 ppm, a 97% reduction, by2006.

However, NO_(x) emissions are an exception, since in the case ofbiodiesel fueling there is a well documented increase of 2-4% in NOxemissions with a blend of 20 vol. % methyl soyate in petroleum dieselfuel such as is described by Graboski and McCormick in an article inProgress in Energy and Combustion Science (1998) vol. 24(2), pp.125-164, hereafter referred to as the ‘Graboski paper’.

Researchers have been looking for the underlying causes of the biodiesel“NOx effect”. Heywood has shown in “Internal Combustion EngineFundamentals”, 1988, McGraw-Hill, New York, p. 864, that advancinginjection timing can lead to an increase in NOx emissions from dieselengines. Several researchers have reported an advance in the fuelinjection timing when biodiesel is being used, such as is described byChoi and coworkers in a 1997 SAE Technical Paper No. 970218 hereafterreferred to as the ‘Choi paper’, and by Monyem and coworkers in anarticle in Transactions of the ASAE (2001) vol. 44(1), pp. 35-42hereafter referred to as the ‘Monyem paper’. It was further demonstratedby the ‘Monyem paper’ that there was a linear dependence on NOx and theactual start of injection, regardless of the fuel used. Szybist andBoehman described in their 2003 SAE Technical Paper No. 2003-01-1039,which is incorporated herein by reference in its entirety, the use of acombination of spray visualization, laser attenuation, fuel linepressure sensing and heat release analysis to study the impact ofbiodiesel blending on the start of injection, end of injection andignition delay in an air-cooled single cylinder direct injection dieselengine. They made comparisons of injection timing and duration fordiesel fuel and a range of biodiesel blends (B20 to B100). Shifts ininjection timing were observed between the fuel blends, amounting to a 1crank angle (CA) degree difference between diesel fuel and purebiodiesel (B100). Combustion studies were also performed to see how theshift in injection timing affected the timing of the combustion process.There was an advance in ignition of up to 4 CA degrees with B100, whichcan be attributed, at least in part, to the advanced injection timing.

Some researchers suggested that differences in the physical propertiesof the biodiesel, such as its increased kinematic viscosity as describedin the ‘Choi paper’ or a higher bulk modulus of compressibility asdescribed in the ‘Monyem paper’, could lead to variations in fuelinjection timing. Biodiesel has indeed a higher viscosity (ca. 4.1 mm²/sat 40° C.) compared to diesel (ca. 2.6 mm²/s at 40° C.). Viscositydirectly influences the amount of fuel that leaks past the plunger inthe fuel pump and the needle in the fuel injection nozzle. Highviscosity fuels, such as biodiesel, lead to reduced fuel losses duringthe injection process compared to lower viscosity fuels, leading to afaster evolution of pressure, and thus an advance in fuel injectiontiming as shown by Tat and Van Gerpen (2003) in their NREL reportNREL/SR-510-31462, p. 114, hereafter referred to as the ‘Tat 2003paper’. Tat and coworkers have previously suggested in an article inJournal of the American Oil Chemists Society (2000) vol. 77(3), pp.285-289, hereafter referred to as the ‘Tat 2000 paper’, that adifference in the bulk modulus of compressibility could be responsiblefor the difference in fuel injection timing; and that the NOx increasewith biodiesel fueling is attributable to an inadvertent advance of fuelinjection timing. The effect of viscosity on fuel injection timingrelative to the effect of bulk modulus is currently unknown.

A diesel fuel injection system was modeled by Rakopoulos and Hountalasand described in their article in Energy Conversion and Management,1996, vol. 37(2), pp. 135-150. The system was separated into fivecontrol volumes. A constant pressure throughout four of the controlvolumes could be assumed at any given time: the pumping chamber,delivery valve chamber, the injector main volume, and the sac volume.For these volumes the only fuel property that was important in modelingthe pressure as a function of time was the bulk modulus ofcompressibility. A less compressible fuel will result in a faster risein pressure in the chamber. Constant pressure throughout the fifthcontrol volume, the fuel line from the pump to the injector, could notbe assumed. The only fuel properties that were important in modeling thepressure as a function of time in the fuel line were the density andspeed of sound, which is dependent on the bulk modulus ofcompressibility. It was not necessary to take fuel viscosity intoaccount to in order to validate the model.

In a similar model of the fuel injection system, a sensitivity analysison the effect that the bulk modulus had on the fuel injection timing wasperformed by Arcoumanis and coworkers and described in their 1997 SAETechnical Paper No. 97034. A 10% increase in the compressibility of thefuel advanced the fuel injection timing 0.5 CA degrees. By comparison,they found that a fuel density difference had a negligible effect on thestart of fuel injection. Fuel viscosity was not found to affect the fuelinjection timing.

Some researchers have reported that certain compositions in biodieselwere more prone to the “NO_(x) effect”. Biodiesel fuel compositiondepends on the feedstock that is subjected to the transesterificationprocess. McCormick and co-workers observed in a paper in EnvironmentalScience and Technology (2001) vol. 35, pp. 1742-1747, hereafter referredto as the ‘McCormick 2001 paper’, that the unsaturated methyl and ethylesters of fatty acids, produced from soybean and linseed oils, yield thehighest NO_(x) increase in a diesel engine. In contrast, the most highlysaturated methyl and ethyl esters of fatty acids, produced from tallowor by hydrogenating the ethyl and methyl esters, yield much lower NOxemissions, in some cases even lower than for diesel fuel. McCormick andco-workers (2001) stated that the fuel chemistry was at the root of thefuel properties and the increased NO_(x) emissions.

Efforts to combat the “biodiesel NOx effect” have included blending ofbiodiesel with various fuel stocks, selection of different biodieselfeedstocks, and use of cetane improvers. McCormick's group showed in a2002 SAE Technical Paper No. 2002-01-1658 hereafter referred to as the‘McCormick 2002 paper’, that a roughly “NO_(x) neutral” B20 (soybeanoil-derived) biodiesel fuel could be obtained by blending with thebiodiesel with: a diesel blend consisting of 46% Fischer-Tropsch dieselfuel, a diesel basestock containing 10% aromatics, 1 vol. %di-tert-butyl peroxide (DTBP), 0.5 vol. % ethyl-hexyl nitrate, or 500ppm by volume of ferrocene. In each blend, it appeared that thereduction in NOx from the base of B20 blended with conventional dieselfuel arose from enhancing the cetane number of the fuel, which each ofthe above cases should provide.

Consequently, the problem remains that biodiesel (B100 and B20)increases NO_(x) emissions from diesel engines. There is still a needfor clean alternate fuels for compression ignition engines in order toreduce NOx emissions. This invention provides a more efficient andeffective method for blending synthetic distillates and biofuels to meetcurrent NO_(x) specification for diesel specification or to formulatebiodiesel blends with reduced NO_(x) emissions as well as very lowsulfur emissions, without altering other important fuel specificationsrelated to injection quality (cetane number).

SUMMARY OF THE INVENTION

These and other needs in the art are addressed in one embodiment by amethod for forming a synthetic environmentally friendly diesel fuel ordiesel fuel blendstock with reduced sulfur and NO_(x) emissions whenused in compression ignition engines. This invention relates to theblending of a synthetic middle distillate derived from synthesis gas anda biodiesel derived from biomass for the production ofenvironment-friendly diesel fuels, which generate very low levels ofNO_(x) and sulfur emissions when used in diesel engines.

The invention relates to a synthetic environmentally-friendly fuel foruse in compression ignition engines, wherein the syntheticenvironmentally-friendly fuel comprises a mixture of a synthetic liquiddistillate and a liquid biofuel, and wherein the syntheticenvironmentally-friendly fuel is characterized by a sulfur level lessthan 20 ppmS; a specific gravity equal to or less than 0.84; a bulkmodulus of compressibility between about 1300 megapascals (MPa) andabout 1600 MPa measured at 15 MPa and 37.8 degrees C. (° C.), and acetane number greater than 55. The environmentally-friendly diesel fuelpreferably has a specific gravity between about 0.78 and about 0.84. Insome embodiments, the synthetic environmentally-friendly fuel comprisesprimarily the synthetic liquid distillate and the liquid biofuel, andmay further contains minor components, such as fuel additives. Inalternate embodiments, the synthetic environmentally-friendly fuelcomprises essentially a mixture of the synthetic liquid distillate andthe liquid biofuel.

As used herein, “biofuel” is defined as a liquid energy source that isderived from agricultural crops or residues or from forest products orbyproducts and can be substituted for liquid or gaseous fuels derivedfrom petroleum or other fossil carbon sources. Biodiesel is one type ofbiofuel.

In preferred embodiments, the environmentally-friendly fuel comprises avolume fraction of the liquid biofuel between about 1 percent and about45 percent. In alternate embodiments, the environmentally-friendly fuelcomprises a liquid biofuel volume fraction between 1 percent and 20percent; or between about 20 percent and 45 percent; or at about 45percent.

Other embodiments for the environmentally-friendly fuels for compressionignition engines include blends of a synthetic distillate with two ormore biofuels from different feedstocks.; blends of two or moresynthetic distillates with a biofuel from one feedstock; or anycombination thereof.

The invention further relates to a method for forming a syntheticenvironmentally-friendly diesel fuel or diesel fuel blendstock, saidmethod comprising blending a liquid biofuel with a synthetic liquiddistillate so as to form a fuel for compression ignition engines, saidfuel being characterized by a bulk modulus of compressibility measuredat 15 MPa and 37.8° C. between about 1300 MPa and about 1600 MPa, acetane number greater than about 55, and wherein theenvironmentally-friendly diesel fuel comprises less than 20 ppm sulfur.The specific gravity of the resulting blend is preferably equal to orlower than 0.84. The method may further include adjusting the volumetricratio of synthetic distillate to biofuel in the fuel so as to meet aspecific gravity of the fuel of less than 0.84.

The synthetic middle distillate preferably is preferably derived from aFischer-Tropsch synthesis. The method for forming a syntheticenvironmentally-friendly diesel fuel may further include the followingsteps: feeding a synthesis gas to a hydrocarbon synthesis reactor,wherein the synthesis gas is reacted under conversion promotingconditions to produce a hydrocarbon synthesis product; optionally,hydroprocessing at least a portion of said the hydrocarbon synthesisproduct to a hydroprocessing unit, wherein the hydrocarbon synthesisproduct is hydroprocessed; fractionating the hydrocarbon synthesisproduct to at least generate a synthetic middle distillate, wherein thesynthetic middle distillate has a boiling range comprising a 5% boilingpoint between about 170° C. and about 210° C. and a 95% boiling pointbetween about 320° C. and about 350° C.

The synthetic distillate utilized in the environmentally-friendly fuelis further characterized by a cetane number greater than 75, by a sulfurcontent less than 10 ppm sulfur; by a paraffin content greater thanabout 90 percent; and by a specific gravity less than about 0.8.

The liquid biofuel in the environmentally-friendly fuel is preferablycharacterized by a sulfur content less than 30 ppm sulfur, and by aspecific gravity greater than about 0.86. The liquid biofuel is derivedfrom biomass such as vegetable oils, animal fats, waste vegetable oils,and/or microalgae oils. Preferably, the biofuel comprises an organiccompound selected from the group consisting of esters of fatty acids,hydrogenated esters of fatty acids, pure vegetable oils, andcombinations thereof. The liquid biofuel more preferably comprisesprimarily alkyl esters of fatty acids, wherein said fatty acids arecharacterized by having between 8 to 22 carbons atoms. The liquidbiofuel is preferably derived by the esterification and/ortransesterification of one or more vegetable oils.

The invention further relates to a method for forming a “NO_(x) neutral”fuel formulation comprising a synthetic liquid distillate and a liquidbiofuel by adjusting the ratio of synthetic liquid distillate to liquidbiofuel to achieve a bulk modulus of compressibility similar to that ofa petroleum diesel formulation.

Additionally, the invention relates to a method for forming a syntheticdiesel fuel formulation comprising primarily of a synthetic distillateand a biofuel within a biofuel volumetric fraction within an optimumrange which comprises a bulk modulus of compressibility in saidsynthetic diesel fuel formulation lower than that of a conventional(crude derived) diesel fuel such that the synthetic diesel fuelformulation generates reduced NO_(x) emissions compared to petroleumdiesel fuel formulations.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter that form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand the specific embodiments disclosed may be readily utilized as abasis for modifying or designing other structures for carrying out thesame purposes of the present invention. It should also be realized bythose skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the invention as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the preferred embodiments of theinvention, reference will now be made to the accompanying drawings.

FIG. 1 illustrates the measured bulk modulus as a function of pressurefor biodiesel (B100), FT diesel, blends thereof (B20-FT and B40-FT), 20%biodiesel blend in petroleum diesel fuel (B20), a ultra-low sulfurdiesel (BP-15), a paraffinic solvent (Norpar®), and soy oil;

FIG. 2 illustrates the relation between the bulk modulus ofcompressibility at 15 MPa and 37.8° C. (100° F.) of a diesel blend withbiodiesel and FT diesel with respect to the biodiesel volume fraction inthe diesel blend;

FIG. 3 illustrates the relation between injection timing (Crank AngleVariation) and bulk modulus of compressibility measured at 6.89 MPa and100° F. for biodiesel (B100), biodiesel blend with baseline diesel(B20), baseline petroleum diesel (BP-15), biodiesel blend with FT diesel(B20-FT) and a paraffin solvent (Norpar®);

FIG. 4 illustrates the relation between specific gravity and bulkmodulus of compressibility of various fuels measured at 6.89 MPa and37.8° C. (100° F.);

FIG. 5 illustrates a high pressure housing for bulk modulusmeasurements; and

FIG. 6 illustrates the schematic diagram of a spray visualizationchamber connected to a direct injection diesel engine with access fordigital imaging and laser attenuation measurements of fuel injectiontiming.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

To more clearly illustrate the present invention, several drawings arepresented. However, no limitations to the current invention should beascertained from the drawings presented herein below.

This invention shows how to use a biodiesel-based fuel in diesel engineswithout incurring the NO_(x) penalty, that should ultimately assist indeploying the use of biodiesels in areas where ozone non-containment isa problem and a hindrance to biodiesel utilization.

Even though there exist physical as well as chemical reasons for thechanges in NO_(x) emissions with fuels, the Applicants believe thattrends in NO_(x) emissions can be related primarily to the bulk modulusof compressibility of the fuel. There exists therefore an optimum rangeof bulk modulus of compressibility for biodiesel blends, which resultsin generating “NO_(x) neutral” biodiesel blends or to formulatebiodiesel blends with lower NO_(x) emissions than conventional dieselfuel.

A related factor to the compressibility of the fuel is the speed ofsound. The speed of sound of a fuel can affect the performance of sometypes of fuel injection systems, reducing the time for the pressurepulses to travel from the injection pump to the injector. All injectionsystems compress more fuel that is injected. The bulk modulus and speedof sound of fuels are related to each other and to fuel density throughmathematical relationships. By measuring a fuel's speed of sound anddensity, the adiabatic bulk modulus can be calculated. A higher bulkmodulus of compressibility, or speed of sound, in the fuel blend, leadsto a more rapid transferal of the pressure wave from the fuel pump tothe injector needle and an earlier needle lift. This property, the bulkmodulus of compressibility, can then be associated with fuel injectionperformance on biodiesel and blends.

Tat and coworkers measured and disclosed the bulk modulus ofcompressibility for biodiesel and petroleum diesel in the ‘Tat 2000paper’ from atmospheric pressure to 35 MPa, and showed that the bulkmodulus of compressibility increased linearly with pressure.Additionally, for the pressure range they studied, the bulk modulus ofbiodiesel was always 5-10% higher than diesel fuel. One can thereforeconclude that biodiesel is less compressible than petroleum diesel.

The accompanying FIG. 1 shows the measured bulk modulus ofcompressibility for some fuel samples: a biodiesel fuel (B100; methylsoyate), a biodiesel blend B20 with a petroleum diesel, an unrefinedsoybean oil, a commercially-available normal paraffin mixture fromC₁₁-C₁₅ (Norpar®-13), a Fischer-Tropsch diesel, two biodiesel blendswith FT diesel (B20-FT; B40-FT), and an ultra-low sulfur baseline dieselfuel (BP-15). Norpar® is the trademark for a line of hydrocarbon fluidswith very high normal paraffin content (>95%) and relatively narrowboiling ranges, commercially available from ExxonMobil corporation. Itwill be understood that other such hydrocarbon fluids can be substitutedtherefor. The bulk modulus of compressibility for B20, B100 and thesoybean oil are higher than that of the baseline diesel fuel, consistentwith the results reported in the ‘Tat 2000 paper’. FIG. 1 also showsthat the bulk modulus of compressibility for B20 was slightly above thatof the baseline diesel. One should note that the “B20” fuel from the‘Graboski paper’ had between 2 and 4% higher NOx emissions than thediesel fuel. We can conclude that the increase in NOx emissions observedfor B20 by Graboski and McCormick in the ‘Graboski paper’ was due to anincrease in the bulk modulus of compressibility of the B20 formulation.

FIG. 1 also shows that the bulk modulus of the B40-FT blend is slightlybelow that of the baseline diesel, and that the B20-FT blend and FTdiesel have significantly lower bulk moduli of compressibility than thebaseline diesel. FIG. 1 therefore shows that FT diesel and blends of FTdiesel comprising up to 40% biodiesel volume fractions are thereforemore compressible than baseline petroleum diesel.

FIG. 2 represents the bulk modulus of compressibility for biodieselblends with FT diesel versus the biodiesel volumetric fraction of theblends, 0% being pure FT diesel and 100% being pure B100. FIG. 2 can beutilized to determine what blend ratio would be equivalent in bulkmodulus to the baseline diesel and what blend ratio would yield a bulkmodulus lower than that of the baseline diesel. A B45-FT blendcorresponds to a bulk modulus of compressibility at 15 MPa of 1580 MPa,equivalent to that of the baseline diesel. Hence the B45-FT blend wouldcomprise a “NOx neutral” biodiesel blend. Accordingly, the biodiesel-FTblends with less than 45% of biodiesel should give a reduced level ofNO_(x) emissions when used in a diesel engine compared to a petroleumdiesel or a B20 blend with petroleum diesel, or B100 biodiesel. Thus,one strategy for combating the biodiesel “NO_(x) effect” is to usehighly paraffinic diesel fuels, such as FT diesel as the dieselbasestock, in such as way that the bulk modulus of compressibility iswithin a desirable range to result in NO_(x) emissions equating currentpetroleum diesel fuel or to result in even lower NO_(x) emissions incompression ignition engines than current petroleum diesel fuel.

The plot in FIG. 2 also indicates that the bulk modulus of methyl soyatebiodiesel blends with FT diesel increases in a linear fashion withrespect to the biodiesel volumetric fraction in the blend. This linearfit of bulk modulus of biodiesel blends and biodiesel fractions shouldbe quite helpful in determining the optimal range of biodiesel/FT dieselratios, once the bulk modulus of compressibility is determined for thepure biodiesel and for pure FT diesel. A correlation can be establishedto calculate the biofuel volume fraction needed in a biofuel/syntheticdistillate blend to make the environmentally-friendly diesel fuel with adesired bulk modulus of compressibility selected within an optimum rangeof bulk moduli of compressibility once the measured bulk moduli ofcompressibility of the two main components of the blends: biofuel andsynthetic distillate are measured. This correlation is shown by Equation(1):

x _(b)=100(B−B _(sd))/(B _(b) −B _(sd))  (1)

wherein B represents the desired bulk modulus of compressibility of thefuel blend at a specific pressure and a specific temperature; B_(sd)represents the measured bulk modulus of compressibility of the syntheticdistillate at the same pressure and temperature settings; B_(b)represents the measured bulk modulus of compressibility of the liquidbiofuel at the same pressure and temperature settings; and x_(b)represents the calculated biofuel volume fraction in the resulting fuelblend to achieve the desired bulk modulus B of compressibility of thefuel blend.

FIG. 3 indicates that there is an advance of fuel injection timing(positive CA change) for the diesel-biodiesel (B20) and pure biodiesel(B100). B100 has the most advance in fuel injection timing and also hasthe highest bulk modulus of compressibility as shown in FIG. 3. Theadvanced in injection timing confirms the work described in the ‘Choipaper’ and the ‘Monyem paper’. The ‘Choi paper’ reported an advance infuel injection timing, 0.6 CA degrees with a 40% volume blend ofbiodiesel with a petroleum diesel. The ‘Monyem paper’ reported anadvance in fuel injection timing, based on the fuel line pressure, of2.3 CA degrees with neat biodiesel, and 0.25 to 0.75 CA degrees with a20% volume blend of biodiesel using a John Deere 4276 DI engine.

FIG. 3 also shows that the purely paraffinic solvent, Norpar®, and the20% blend of B100 and FT diesel (B20-FT) retard the fuel injectiontiming by 0.5 CA and 0.1 CA respectively, compared to the baselinediesel (BP-15). Norpar® has the most retardation in fuel injectiontiming and also has the lowest bulk modulus of compressibility as shownin FIG. 3. This retardation in injection timing associated with theparaffinic mixture gives support to the proposition that variation ininjection timing due to the lower bulk modulus of compressibility is acontributing factor in the reductions in NO_(x) emissions observed withFT diesel fuels. And, the retarded injection timing provides anexplanation for the reduced NO_(x) emissions measured by McCormick'sgroup and reported in the ‘McCormick 2002 paper’ when they blended FTdiesel with conventional diesel to produce a partly paraffinic base fora B20 blend.

The data in FIGS. 1 and 3 for bulk modulus and injection timing showthat there is a trend on which one can base a judgment about thepotential impact of a fuel on injection timing and emissions. There isan increase in bulk modulus with increasing density. FIG. 4 shows thespecific gravity of various fuels versus their respective bulk modulus,and the specific gravity of the fuels is correlated directly with thebulk modulus. The fuels represented in FIG. 4 include data from severalpublished sources: the ‘McCormick 2001 paper’; the ‘Tat 2003 paper’, andan article by Ofner et al. in Inst. Mech. Engr. J. (1996) “A fuelinjection system concept for dimethyl ether,” vol. 22, pp. 275-287hereafter referred to as the ‘Ofner paper’. The ‘Tat 2003 paper’presented a survey of the bulk moduli of the various methyl and ethylesters at 6.89 MPa (1000 psi), that are common in biodiesel fuels albeitmeasured at a slightly higher temperature of 40° C. (104° F.). Thelowest density in FIG. 4 represents dimethyl ether with the lowest bulkmodulus (B˜450 MPa at 3.4 MPa pressure), which originates from the‘Ofner paper’. Hence, FIG. 4 shows a definitive trend between bulkmodulus and fuel density, since the bulk modulus generally seems toincrease with an increase in fuel specific gravity. That trend can berepresented by a linear relationship as shown in Equation (2) with acorrelation coefficient R²=0.925

B=−2,666+4,966*SG  (2)

where B is the bulk modulus of compressibility in MPa and SG is thespecific gravity. Although there seems to be a correlation betweenspecific gravity and bulk modulus of compressibility for these fuels, itis to be noted however that fuels of similar specific gravity can differby 20 MPa or more, or 50 MPa or more, or sometimes 100 MPa or more inbulk modulus of compressibility, and that the bulk modulus ofcompressibility of a fuel could be lower than that of a fuel with alower specific gravity, as illustrated by the scatter of data points forthe various fuels with specific gravity between about 0.84 and 0.89.Similarly, the actual bulk modulus for dimethyl ether (with the lowestspecific gravity) is much lower than would have been predicted withEquation (2).

The NO_(x) emissions trends observed in the ‘McCormick 2001 paper’ forthe speciated biodiesel constituents and various biodiesel feedstockscan be explained, in light of the present work, on the basis of thevariation of the bulk modulus of the fuels, consistent with theobservations by Van Gerpen and co-workers disclosed in the ‘Monyempaper’ and the ‘Tat 2002 paper.’ These same observations have relevanceto the formulation of reformulated diesel fuels, FT diesel fuels,biodiesel fuels (B20, B100, etc.) and blends thereof.

These observations on the correlation of bulk modulus with density areconsistent with the historical literature on the bulk modulus ofhydrocarbons. Bridgman in his 1958 book “The Physics of High Pressure”,published by G. Bell and Sons (London), pp. 116-149 described themethodologies for testing the compressibility of fluids and presented asummary of results that were available at that time for various fluids,including normal alkanes, iso-alkanes, alcohols, halogenated compoundsand water. Bridgman asserted that the compressibility of fluids at lowerpressures was due to consumption of free space between the looselypacked molecules. At higher pressures, the compressibility is less andis due to compression of the molecules themselves that would be opposedby intermolecular repulsion. Thus, the bulk modulus should increase withincreasing pressure as the resistance to further compression increases.

Similar trends in compressibility of hydrocarbons were observed byCutler and coworkers in their paper in the Journal of Chemical Physics(1958) vol. 29, pp. 727-740 hereafter referred to as the ‘Cutler paper’,who considered a variety of pure hydrocarbons including normal alkanesfrom C₁₂ to C₁₈, branched alkanes, cycloalkanes and aromatic compounds.The ‘Cutler paper’ disclosed that compressibility was greatest fornormal alkanes, which have a less rigid structure, and decreased withincreasing rigidity of molecular shape. Thus, compressibility increasedas molecular structure varied from multi-ring aromatic, to aromatic, tocycloalkane, to branched alkane, and to straight-chain alkane. Since thebulk modulus is inversely related to the compressibility, the trend forbulk modulus would be to increase with increasing rigidity. FollowingBridgman's logic, the bulk modulus would also increase with increasingdensity, because a more dense fluid would possess less of the free spaceto be consumed during compression.

For the various biodiesel fuel stocks considered in the ‘McCormick 2001paper’, there appears to be a strong correlation between NO_(x)emissions and density. An underlying reason for the trend of increasingNO_(x) with increasing density (and Iodine Number, which is anindication of the degree of unsaturation) is that as the density of thebiodiesel feedstock increases, its bulk modulus increases and leads toadvanced injection timing.

The Applicants believe that the higher bulk modulus of compressibilityof vegetable oils and their methyl esters leads to advanced injectiontiming. Advanced injection timing has been shown in the literature tocontribute to the NO_(x) emissions increase with the use of biodieselfuel. An opposite trend, a retarding of injection timing, is observedwith paraffinic mixtures because they have a lower bulk modulus ofcompressibility than conventional diesel fuels. This supports theobservation that paraffinic fuels such as Fischer-Tropsch diesel fuelsyield lower NO_(x) emissions. Thus, the biodiesel “NOx effect” can beattributed to variations in the bulk modulus of the fuel or fuel blend,and these effects correlate for biofuels and paraffinic fuels quite wellwith fuel density.

The present work also shows that a 45 vol. % blend of a methyl soyatebiodiesel and a FT diesel displays the same bulk modulus ofcompressibility as a ultra-low sulfur petroleum diesel fuel. Thus, onestrategy for combating the biodiesel “NOx effect” is to use highlyparaffinic diesel fuels, such as FT diesel as the diesel basestock.

Blends of Biofuel and Synthetic Distillate

The invention relates to a synthetic environmentally-friendly fuel ordiesel fuel blendstock (generally referred to collectively herein as a“fuel”) for use in compression ignition engines, the synthetic fuelcomprising a mixture of a synthetic liquid distillate and a liquidbiofuel, said mixture being characterized by a sulfur level less than 20ppm S; a specific gravity equal to or less than 0.84; a bulk modulus ofcompressibility between about 1300 MPa and about 1600 MPa measured at 15MPa and 37.8° C., and a cetane number greater than 55. Theenvironmentally-friendly diesel fuel preferably has a specific gravitybetween about 0.78 and about 0.84. Preferably, theenvironmentally-friendly diesel fuel has a cetane number greater thanabout 60; more preferably, a cetane number greater than about 65.

In some embodiments, the synthetic environmentally-friendly fuelcomprises primarily at least one synthetic liquid distillate and atleast one liquid biofuel (i.e., greater than 90% volume comprises bothcomponents), and may further contain minor components, such as fueladditives.

In alternate embodiments, the synthetic environmentally-friendly fuelcomprises essentially of a mixture of at least one synthetic liquiddistillate and at least one liquid biofuel (i.e., greater than 95%volume comprises both components).

Since biofuels tend to increase the bulk modulus of compressibility, andthe paraffinic fuels tend to decrease the bulk modulus ofcompressibility, there exists an optimum biofuel volume fraction in thesynthetic fuel blends that yields a bulk modulus of compressibilityabout equal to that of a petroleum diesel fuel so as to achieve “NO_(x)neutral” synthetic fuels. The preferred biodiesel blends of the presentinvention comprise a synthetic middle distillate, such that the NO_(x)effect is mitigated by the selection of an optimum range of bulk modulusof compressibility for each biodiesel blend. This optimum range of bulkmodulus of compressibility is primarily dependent on a specific range ofvolumetric ratios of synthetic middle distillate to biodiesel in thebiodiesel blend, and the respective bulk modulus of compressibility ofpure synthetic middle distillate and pure biodiesel.

The environmentally-friendly fuel preferably comprises a volume fractionof liquid biofuel between about 1 percent and about 45 percent. In thesepreferred embodiments, the environmentally-friendly fuel comprises abulk modulus of compressibility measured at 15 MPa and 37.8° C. betweenabout 1300 MPa and about 1600 MPa, and a cetane number greater than 55.

In alternate embodiments, the environmentally-friendly fuel comprises aliquid biofuel volume fraction between 1 percent and 20 percent; orbetween about 20 percent and 45 percent; or at about 45 percent.

The environmentally-friendly fuel comprising a volume percent of liquidbiofuel between 1 percent and 20 percent has a bulk modulus ofcompressibility measured at 15 MPa and 37.8° C. between about 1300 MPaand about 1500 MPa, and a cetane number greater than 70.

In alternate embodiments, the environmentally-friendly fuel comprises avolume percent of liquid biofuel between about 20 percent and 45 percenthas a bulk modulus of compressibility measured at 15 MPa and 37.8° C.between about 1500 MPa and about 1600 MPa; a cetane number greater thanabout 60; and a specific gravity between about 0.8 and about 0.84.

In alternate embodiments, the environmentally-friendly fuel comprises avolume percent of liquid biofuel of about 45 percent so that the fuelhas a bulk modulus of compressibility measured at 15 MPa and 37.8° C.between about 1560 MPa and about 1600 MPa.

In additional embodiments, the environmentally-friendly fuel comprises abiofuel to synthetic distillate volumetric ratio between 0.01 and about0.45; or between 0.01 and about 0.2; between 0.2 and about 0.45; or atabout 0.45.

In preferred embodiments, the synthetic distillate is a FT dieseldistillate. FT Diesel distillate can be combined with the biofuel in anyratio suitable to reduce the bulk modulus of compressibility of theblend to a desired value so as to produce a diesel fuel with “NOxneutral” effect which produced reduced NOx emissions when used in adiesel engine compared to a conventional petroleum diesel.

In alternative embodiments, FT diesel distillate can be combined withbiofuel to reduce the density of the resulting blend for any desiredreason. For instance, a biofuel can be combined with FT dieseldistillate to satisfy density specifications for a diesel fuel.Typically, these specifications determine allowable uses of diesel fuel,classifications of diesel fuel, and the like, which are all well known.Examples of allowable uses of a diesel fuel include on-road use,off-road use, and the like. For instance, regulations may require thatthe diesel fuel have a density within a specified range to qualify as anon-road use diesel fuel. The regulations may also require the dieselfuel to comprise other properties, such as cetane number, sulfurcontent, aromatics content, and the like, within a specified range toqualify as the on-road use diesel fuel. An example of classificationsfor diesel fuels includes specifications for a No. 2 diesel fuel. Theseclassifications are well known and include World-Wide Fuel Charterclassifications, ASTM classifications, European classifications, and thelike. For instance, the December 2002 World-Wide Fuel Charter recommendsa density range measured at 15° C. of about 820 kg/m³ to about 850 kg/m³(the minimum limit can be relaxed to 800 kg/m³ when ambient temperaturesare below −30° C.) for a No. 2 diesel fuel. To bring an off-specsynthetic mixture such as a FT diesel and a biofuel to within thedensity specifications of the 2002 World-Wide Fuel Charterspecifications for a No. 2 diesel fuel, FT diesel distillate can becombined with a biofuel comprising mono-alkyl esters of fatty acids toform a diesel product in a desired ratio to bring the density of thediesel product within the specification limits of 815 kg/m³ to about 850kg/m³, as long as the bulk modulus of compressibility is within or belowthe “NO_(x) neutral” zone for diesel.

The FT diesel distillate can be further blended with the biofuel toachieve a specific gravity of the blend below 0.84 and to adjust atleast one other property of the blend, wherein the other propertiesinclude the cetane number, lubricity, iodine number, viscosity, and thelike.

With the FT diesel distillate preferably having a density at 15° C. fromabout 0.76 g/cm³ to about 0.80 g/cm³, more preferably between about 0.77g/cm³ to about 0.79 g/cm³, FT diesel distillate can be combined with abiofuel having a higher bulk modulus of compressibility than FT dieseldistillate to produce a synthetic diesel fuel having a bulk modulus ofcompressibility lower than that of the biofuel. The FT diesel distillatecan be combined with biofuel by any known method. In the presentembodiment, the FT diesel distillate is combined with the biofuel in avessel.

Preferably, the environmentally-friendly fuel has a sulfur level lessthan 15 ppm S; more preferably less than 10 ppm S; still more preferablyless than 5 ppm S.

Preferably, the environmentally-friendly fuel for compression ignitionengines has a boiling range with a 5% boiling point between about 320°F. and 350° F. (about 160-177° C.) and a 95% boiling point between about600° F. and 650° F. (about 315-343° C.).

In another embodiment, the environmentally-friendly synthetic liquidfuel has a boiling range having a 5% boiling point between about 340° F.and about 410° F. (or between about 170° C. and about 210° C.) and a 95%boiling point between about 570° F. and about 645° F. (or between about300° C. and about 340° C.).

Alternatively, the environmentally-friendly synthetic liquid fuel has aboiling range having a 5% boiling point between about 355° F. and about420° F. (or between about 180° C. and about 215° C.) and a 95% boilingpoint between about 600° F. and 650° F. (or between about 315° C. and343° C.).

Biofuel

The biofuel or blendstock should comprise one or more organic compoundsselected from the group consisting of esters of fatty acids,hydrogenated esters of fatty acids, pure vegetable oils, andcombinations thereof. The liquid biofuel more preferably comprisesprimarily alkyl esters of fatty acids, each having between 8 to 22carbon atoms. Preferred esters of fatty acids are methyl esters, ethylesters, or combinations thereof, of fatty acids, said fatty acidscomprising between 8 and 22 carbon atoms. Examples of suitable fattyacid esters are methyl laurate, methyl palmitate, methyl stearate, ethylstearate, methyl oleate, methyl linoleate, ethyl linoleate, methyllinolenate, and the like.

The biofuel preferably comprises a density higher than about the densityof synthetic distillate. The biofuel preferably comprises mono-alkylesters of fatty acids which include products of esterification and/ortransesterification of vegetable oils, animal fats, and yellow grease,said products comprising very small amounts of glycerol and/or alcohol.More preferably, the liquid biofuel comprises an alkyl ester of one ormore vegetable oils selected from the group consisting of canola oil,cotton oil, sunflower oil, coconut oil, palm oil, soya oil, andcombinations thereof. Still more preferably, the liquid biofuelcomprises alkyl esters of fatty acids produced from soybean oil and/orcanola oil, wherein the alkyl esters are methyl esters, ethyl esters, orcombinations thereof. In some embodiments, the liquid biofuel comprisesmethyl soyate.

The liquid biofuel can be characterized by a bulk modulus ofcompressibility measured at 15 MPa and 37.8° C. greater than about 1600MPa, preferably between about 1650 MPa and about 2100 MPa.

Furthermore, the biofuel has a very low sulfur content, i.e., less than50 ppm sulfur, preferably less than 30 ppm sulfur, preferably less than10 ppm sulfur. Biodiesel comprising canola oil and/or canola alkylesters, may have a sulfur content slightly higher than from otherfeedstocks. The biofuel should also have very low aromatic and nitrogencontents.

In addition, the liquid biofuel is preferably substantially free ofglycerin, i.e., the total glycerin content of the liquid biofuel shouldbe less than 0.24 percent by weight. The ‘free’ glycerol content ispreferably less than 0.02 percent by weight. The free and total glycerincan be measured employing the ASTM method D-6584 “Test Method forDetermination of Free and Total Glycerine in B-100 Biodiesel MethylEsters by Gas Chromatography”. In some embodiments, the liquid biofuelcomprises a substantially glycerin-free product of the esterification ofsoya oil, canola oil or mixtures thereof.

Preferably, the liquid biofuel has a specific gravity greater than about0.86; more preferably between about 0.86 and about 0.91; still morepreferably between about 0.86 and about 0.89.

The liquid biofuel is not meant to comprise essentially alcohols, suchas methanol and ethanol, which also can be derived from biomass(renewable resources). The liquid biofuel preferably should contain avery low content of ‘free’ (i.e., unbound) alcohol molecules, i.e., lessthan about 10 percent by weight.

In the case when the method of making the liquid biofuel includes theuse of an alcohol during the esterification process described later, theflash point for biodiesel is used as the mechanism to limit the level ofun-reacted alcohol remaining in the finished fuel. The flash pointspecification for biodiesel is intended to be 100° C. minimum. Typicalvalues are over 160° C. Due to high variability with the ASTM MethodD-93 “Standard Test Methods for Flash-Point by Pensky-Martens Closed CupTester” as the flash point approaches 100° C., the flash pointspecification has been set at 130° C. minimum to ensure an actual valueof 100° C. minimum. The flash point of the liquid biofuel is preferablygreater than 100° C.; more preferably greater than 130° C.

The liquid biofuel preferably has a boiling range with an initialboiling point between about 300° C. and about 330° C. and a finalboiling point between about 350° C. and about 370° C. according to theASTM distillation method D-86 “Standard Test Method for Distillation ofPetroleum Products at Atmospheric Pressure”; or a boiling range with aninitial boiling point between about 310° C. and about 350° C. and afinal boiling point between about 400° C. and about 480° C. according tothe ASTM vacuum distillation method D-1160 “Standard Test Method forDistillation of Petroleum Products at Reduced Pressure”.

The kinematic viscosity at 40° C. of the liquid biofuel can be betweenabout 1.9 mm²/s (cSt) and about 6 cSt, but preferably between 3 cSt and6 cSt. The kinematic viscosity at 40° C. is preferably measured by theASTM method D-445 “Standard Test Method for Kinematic Viscosity ofTransparent and Opaque Liquids (the Calculation of Dynamic Viscosity)”.

The cetane number of the liquid biofuel is preferably greater than 43;more preferably between about 45 and about 65; still more preferablybetween about 50 and about 60. The cetane number is preferably measuredby the ASTM method D-613 “Standard Test Method for Cetane Number ofDiesel Fuel Oil”.

The density at 15° C. of the liquid biofuel is preferably between about0.86 g/cm³ and about 0.91 g/cm³; more preferably between about 0.87g/cm³ and about 0.89 g/cm³. The density at 15° C. can be measured by theASTM method D-4052 “Standard Test Method for Density and RelativeDensity of Liquids by Digital Density Meter”.

The specific gravity of the liquid biofuel is preferably between about0.86 and about 0.91; more preferably between about 0.87 and about 0.89as measured by the ASTM method D-1298 “Standard Test Method for Density,Relative Density (Specific Gravity), or API Gravity of Crude Petroleumand Liquid Petroleum Products by Hydrometer Method”.

In preferred embodiments, the liquid biofuel meets the specifications ofthe ASTM method D-6751 specifications “Standard Specification forBiodiesel Fuel (B100) Blend Stock for Distillate Fuels”.

Methods of Synthesizing Biofuel

The liquid biofuel is preferably manufactured from vegetable oils,animal fats, waste vegetable oils (such as recycled restaurant greases,called yellow grease) and microalgae oils, or any combination thereof.Methods of preparation of biodiesel are well known. The feedstocks canbe transformed into biodiesel using a variety of esterification ortransesterification technologies. Oils and fats are composed principallyof triglycerides, composed of three long-chain fatty acids of 8 to 22carbons attached to a glycerol backbone, and free fatty acids, whichfatty acid chains break off the triglycerides.

The biofuel is preferably synthesized by esterification and/ortransesterification of one or more feesdstocks selected from the groupsconsisting of vegetable oils (e.g., canola oil, soybean oil, linseedoil, and the like), animal fats (e.g., beef tallow, pork lard), wastevegetable oils (e.g., yellow grease), and microalgae oils.

Biodiesel feedstocks are classified based on the free fatty acid (FFA)content: refined oils (such as refined soybean and canola oils with lessthan 1.5% FFA), low FFA yellow greases and animal fats (<4% FFA), andhigh FFA yellow greases and animal fats (>20%). The production ofbiodiesel from low FFA fats and oils comprises a base catalyzedtransesterification, wherein the triglycerides are transformed intobiodiesel and glycerine under base conditions. The production ofbiodiesel from high FFA fats and oils comprises an acid esterification,wherein FFA are reacted with an alcohol (usually ethanol or methanol) inthe presence of an acid (such as sulfuric acid) to form fatty esterssuch as ethyl or methyl esters. The esterification reaction is thenfollowed by a transesterification.

The liquid biofuel comprising alkyl esters is preferably produced by thebase catalyzed reaction because it is the most economic for severalreasons: low temperature (65° C. or 150° F.) and pressure (20 psi)processing; high conversion (98%) with minimal side reactions andreaction time; direct conversion to methyl ester with no intermediatesteps; and no need for expensive materials of construction. The generalprocess is depicted as follows: a fat or oil is reacted with an alcohol,like methanol or ethanol, in the presence of a catalyst to produceglycerin and alkyl esters (i.e., biofuel). The alcohol is charged inexcess to assist in quick conversion and recovered for reuse. Thecatalyst is usually sodium or potassium hydroxide which has already beenmixed with the methanol.

An overview of present processes from various biodiesel feedstocks wasdescribed by Kinast in his Final Report to USDOE/National RenewableEnergy laboratory, contact ACG-7-15177-02, September 1999. Additionalprocesses for making biofuel are described by Canakci and Van Gerpen intheir paper from Transactions of the American Society of AgriculturalEngineers (2001) vol. 44, No. 6, pp. 1429-1436; as well as in theirpaper from Transactions of the American Society of AgriculturalEngineers (1999) vol. 42, No. 5, pp. 1203-12; and by Freedman andcoworkers in an article in the Journal of the American Oil Chemists'Society (1984) vol. 61, p. 1638.

Synthetic Distillate

The synthetic liquid distillate preferably has a boiling range having a5% boiling point between about 170° C. and about 210° C. and a 95%boiling point between about 320° C. and about 350° C. These boilingpoints are based on the method ASTM D-86 from the American Society forTesting and Materials.

The synthetic liquid distillate preferably has a density at 15° C. offrom about 0.76 g/cm³ to about 0.80 g/cm³; and more preferably betweenabout 0.77 g/cm³ and about 0.79 g/cm³; most preferably at about 0.78g/cm³.

In addition, the synthetic liquid distillate is characterized by aparaffin content greater than about 90 percent, preferably greater thanabout 95 percent. The synthetic liquid distillate should have a sulfurcontent of less than about 10 ppm sulfur; preferably less than about 5ppm sulfur; more preferably less than 1 ppm sulfur; still morepreferably less than about 0.1 ppm sulfur. Moreover, the syntheticliquid distillate preferably has an aromatics content of less than about1 percent by weight.

Furthermore, the synthetic liquid distillate preferably has a cetanenumber greater than about 70, preferably greater than about 75. It is tobe understood that the synthetic liquid distillate is not limited to theabove-identified property values but can include higher or lower valuesdepending on factors such as the synthesis conditions and thehydroprocessing scheme and conditions used to produce it.

In some embodiments, the synthetic liquid distillate comprises verysmall amounts of olefins, i.e., less than 10 percent by weight,preferably less than 5 percent by weight. In alternate embodiments, thesynthetic liquid distillate is characterized by a Bromine number of lessthan about 0.1 g/100 g as measured by the ASTM method D-1159 “StandardTest Method for Bromine Numbers of Petroleum Distillates and CommercialAliphatic Olefins by Electrometric Titration”.

Most of the synthetic liquid distillate used in theenvironmental-friendly fuel is preferably generated by a Fischer-Tropschsynthesis described herein.

Methods of Preparing Synthetic Distillate by Fischer-Tropsch Process

A syngas feed is fed to hydrocarbon synthesis reactor. Syngas compriseshydrogen, or a hydrogen source, and carbon monoxide. Hydrocarbonsynthesis reactor comprises any reactor in which hydrocarbons areproduced from syngas by Fischer-Tropsch synthesis, alcohol synthesis,and any other suitable synthesis. Hydrocarbon synthesis reactor ispreferably a Fischer-Tropsch reactor. Preferably, the hydrogen isprovided by free hydrogen, although some Fischer-Tropsch catalysts havesufficient water gas shift activity to convert some water (and CO) tohydrogen (and CO₂) for use in the Fischer-Tropsch synthesis. It ispreferred that the molar ratio of hydrogen to carbon monoxide in syngasfeed 60 be greater than 0.5:1 (e.g., from about 0.67 to about 2.5).Preferably, when cobalt, nickel, and/or ruthenium catalysts are used,syngas feed contains hydrogen and carbon monoxide in a molar ratio ofabout 1.4:1 to about 2.3:1. Preferably, when iron catalysts are used,syngas feed 60 contains hydrogen and carbon monoxide in a molar ratiobetween about 1.4:1 and about 2.2:1. Syngas feed may also contain carbondioxide. Moreover, syngas feed should contain only a low concentrationof compounds or elements that have a deleterious effect on the catalyst,such as poisons. For example, syngas feed may need to be pretreated toensure that it contains low concentrations of sulfur or nitrogencompounds such as hydrogen sulfide, hydrogen cyanide, ammonia andcarbonyl sulfides.

Syngas feed is contacted with the catalyst in a reaction zone.Mechanical arrangements of conventional design may be employed as thereaction zone including, for example, fixed bed, fluidized bed, slurrybubble column or ebullating bed reactors, among others. Accordingly, thepreferred size and physical form of the catalyst particles may varydepending on the reactor in which they are to be used. A slurry bedreactor with catalyst particles with a weight size average between 30and 150 microns is preferred. The catalyst in the reaction zone forhydrocarbon synthesis preferably comprises a catalytically active metalselected from the group consisting of cobalt, iron, ruthenium, andcombinations thereof. More preferably, the catalyst in the reaction zonefor hydrocarbon synthesis preferably comprises cobalt as onecatalytically active metal. The catalyst may further comprise at leastone promoter suitable for increasing the selectivity, stability, and/oractivity of the reduced catalyst. Suitable promoters are preferablyselected from the group consisting of ruthenium, rhenium, platinum,palladium, boron, manganese, magnesium, silver, lithium, sodium, copper,potassium, and combination thereof. The reduced catalyst may besupported or unsupported. The support for a supported catalystpreferably includes an inorganic oxide such as silica, alumina, titania,or any combination thereof.

The Fischer-Tropsch reactor is typically run in a continuous mode. Inthis mode, the gas hourly space velocity through the reaction zonetypically may range from about 50 to about 10,000 hr⁻¹, preferably fromabout 300 hr⁻¹ to about 2,000 hr⁻¹. The gas hourly space velocity isdefined as the volume of reactants per time per reaction zone volume.The volume of reactant gases is preferably at but not limited tostandard conditions of pressure (101 kPa) and temperature (0° C.). Thereaction zone volume is defined by the portion of the reaction vesselvolume where the reaction takes place and which is occupied by a gaseousphase comprising reactants, products and/or inerts; a liquid phasecomprising liquid/wax products and/or other liquids; and a solid phasecomprising catalyst. The reaction zone temperature is typically in therange from about 160° C. to about 300° C. Preferably, the reaction zoneis operated at conversion promoting conditions at temperatures fromabout 190° C. to about 260° C.; more preferably, from about 205° C. toabout 230° C. The reaction zone pressure is typically in the range ofabout 80 psia (552kPa) to about 1,000 psia (6,895 kPa), more preferablyfrom 80 psia (552 kPa) to about 800 psia (5,515 kPa), and still morepreferably, from about 140 psia (965 kPa) to about 750 psia (5,170 kPa).Most preferably, the reaction zone pressure is from about 250 psia(1,720 kPa) to about 650 psia (4,480 kPa).

The product of hydrocarbon synthesis reactor primarily compriseshydrocarbons. Hydrocarbon synthesis product may also comprise olefins,alcohols, aldehydes, and the like. Hydrocarbon synthesis productprimarily comprises paraffins (more than 80% paraffins).

The hydrocarbon synthesis process should also comprise a fractionator inorder for the product of hydrocarbon synthesis reactor to be separatedinto various fractions, including a gasoline fraction and middledistillate fractions (including diesel fraction). Methods offractionation are well known in the art, and the feed to thefractionator can be separated by any suitable fractionation method. Thefractionator preferably includes an atmospheric distillation column.

The method for making the synthetic distillate may further comprisefeeding the hydrocarbon synthesis product to a hydroprocessing unit,wherein the hydrocarbon synthesis product is hydroprocessed to produce ahydroprocessed product; fractionating the hydroprocessed product toproduce a treated distillate; and combining the treated distillate witha liquid biofuel to produce a fuel, wherein the fuel has a bulk moduluswithin an optimum range and a cetane number greater than 55.

Hydrocarbon synthesis product in part or in totality is preferablyfurther hydroprocessed in order to generate an acceptable yield ofliquid fuels such as gasoline and diesel. Hydroprocessing could be doneon the totality or a portion of the hydrocarbon synthesis product.Hydroprocessing could comprise hydrotreatment, hydrocracking,hydroisomerization, dewaxing, or any combination thereof.

In some embodiments, the hydroprocessing comprises a hydrotreatment toreduce the olefin content of the distillate so that the Bromine number(related to unsaturation of carbon-carbon bonds) is less than 0.1 g/100g as measured for example by ASTM method D-1159 “Standard Test Methodfor Bromine Numbers of Petroleum Distillates and Commercial AliphaticOlefins by Electrometric Titration”.

The hydrotreatment should convert unsaturated hydrocarbons (such asolefins) to saturated hydrocarbons (such as alkanes). The hydrotreatmentcan take place over hydrotreating catalysts. The hydrotreating catalystscomprise at least one of a group VIB metal, such as molybdenum andtungsten, or a group VIII metal, such as nickel, palladium, platinum,ruthenium, iron, and cobalt. The nickel, palladium, platinum, tungsten,molybdenum, ruthenium, and combinations thereof are typically highlyactive catalysts, and the iron and cobalt are typically less activecatalysts. The hydrotreatment is preferably conducted at temperaturesfrom about 140° C. to about 315° C. Other operating parameters ofhydrotreatment may be varied by one of ordinary skill in the art toaffect the desired hydrotreatment. For instance, the hydrogen partialpressure is preferably between about 690 kPa and about 6,900 kPa, andmore preferably between about 2,060 kPa and about 3,450 kPa. Moreover,the liquid hourly space velocity is preferably between about 1 hr⁻¹ andabout 10 hr⁻¹, more preferably between about 0.5 hr⁻¹ and about 6 hr⁻¹,and most preferably between about 1 hr⁻¹ and about 5 hr⁻¹.

Alternatively or in addition, the hydroprocessing may comprise ahydrocracking step to convert heavy paraffins to lighter paraffins.Methods of hydrocracking are well known in the art, and hydrocracking ofheavy distillate (such as wax) can include any suitable method. Thehydrocracking preferably takes place over a platinum catalyst attemperatures from about 260° C. to about 400° C. and at pressures fromabout 3,550 kPa to about 10,440 kPa. The heavy distillate is preferablyfed to a hydrocracker where its components are cracked into smallerhydrocarbon molecules, wherein a good portion of the cracked moleculesare within the boiling range of diesel. The effluent of the hydrocrackeris preferably recycled to a fractionator so the heavy hydrocarbons arerecycled close to extinction.

Alternatively or in addition, the hydroprocessing may comprise ahydroisomerization step to convert paraffins to more branched paraffins,so as to generate a synthetic distillate with at least one improvedcold-flow property (such as lower pour point). It may be desirable tohydroisomerize during the hydrocracking step so as to form branchedhydrocarbons, such as convert linear paraffins to isomers of paraffins(isoparaffins or branched paraffins); and/or convert monobranchedparaffins to dibranched paraffins. Isoparaffins are known to improvecold flow properties in FT diesel, so increasing the relative amount ofbranched hydrocarbons in FT diesel should yield a diesel with decreased(improved) pour point. Paraffin isomerization catalysts and processesthat can be used for producing low pour point diesel fuel can be foundin U.S. Pat. Nos. 4,710,485; 4,589,138; 4,459,312; 5,149,421; 5,282,958,each of which is incorporated herein by reference in its entirety.Additional isomerization processes are described by Taylor and coworkersin Applied Catalysis A: General (1994) vol. 119, pp. 121-138.

To further illustrate various illustrative embodiments of the presentinvention, the following example is provided.

Example

The interaction between the bulk modulus of compressibility of variousfuel samples and their effect on fuel injection timing was examined. Thefuels considered ranged from biodiesel B100 (as methyl soyate, i.e., themethyl ester of soybean oil), unrefined soybean oil, paraffinic solvent,Fischer-Tropsch derived diesel, and ultra low sulfur diesel fuel. Boththe impact on injection timing and the variation in the bulk modulus ofcompressibility were measured so that correlation between fuelcomposition, fuel properties and injection timing could be observed andquantified.

Two different experimental systems were used: a high-pressureviscometer, capable of measuring the bulk modulus of compressibilitywith the use of a pycnometer; and a highly instrumented, single-cylinderdirect injection (DI) diesel engine, with an accompanying sprayvisualization chamber. The bulk modulus of various fuels was measured,and corresponding measurements were made of the impact of the fuel onthe injection timing in the DI diesel engine.

High-Pressure Viscometer

The high-pressure viscometer instrument was developed for studies of theviscosity and bulk modulus of hydraulic fluids that contained dissolvedgases and described by J. A. O'Brien in h is 1963 M. S. Thesis from PennState University, PA, entitled “Precise Measurement of Liquid BulkModulus”. The operation for this device was based on the followingprinciple: when a fluid is exposed to higher pressures, the fluid has areduction in volume.

The isothermal bulk modulus of compressibility, B_(T), and theisentropic bulk modulus of compressibility, B_(S), are defined byEquations (3) and (4) respectively:

$\begin{matrix}{{B_{T} \equiv {- {v\left( \frac{\partial P}{\partial v} \right)}_{T}}} = {\rho \left( \frac{\partial P}{\partial\rho} \right)}_{T}} & (3) \\{{B_{S} \equiv {- {v\left( \frac{\partial P}{\partial v} \right)}_{S}}} = {\rho \left( \frac{\partial P}{\partial\rho} \right)}_{S}} & (4)\end{matrix}$

where ν is the specific volume and ρ is the density. The isentropic bulkmodulus of compressibility is related to the speed of sound, α, by theEquation [5]:

$\begin{matrix}{a = \sqrt{\left( \frac{\partial P}{\partial\rho} \right)_{S}}} & (5)\end{matrix}$

The experimental approach used in this experiment yielded a measurementfor the isothermal bulk modulus of compressibility, B_(T), which willsimply be referred to as B. The governing Equation (6) for thecalculation of bulk modulus is:

$\begin{matrix}{B = {\left( {P - {Po}} \right)\frac{Vo}{\left( {{Vo} - V} \right)}}} & (6)\end{matrix}$

where B is the isothermal bulk modulus, P is the measured pressure, Pois atmospheric pressure, Vo is the volume of the sample at atmosphericpressure and V is the volume at the new pressure.

FIG. 5 shows a schematic diagram of the closed-bottom pycnometer andhousing used in these studies. The measurement equipment 200 comprised amodified 21-R-30 Stainless Jerguson gauge 210 capable of handlingpressures up to 4000 psi. Two panels with viewing windows 220 allowedfor viewing of the sample. Each window glass had two gaskets, one oneither side, to ensure a tight seal on the chamber. For pressures in therange of 0-1000 psi, a direct connection to a helium gas cylinderprovided the necessary pressure via helium inlet 230. For pressuresabove 2000 psi, a 4.5-liter Aminco hydrogenation bomb (not shown) wasfilled with helium, and oil was pumped into the bomb to achievepressures up to 16,000 psi. A constant temperature bath kept thepressure cell at a temperature of 100° F. Bulk modulus was measured viaa change in height in the capillary 240 within the pycnometer tube 250,as the pressure in the cell was varied. The fuel sample was placed inreservoir 260 of the pycnometer tube 250. N-Octadecane was used as acalibration standard.

Direct Injection Diesel Engine and Spray Chamber

A Yanmar L40 AE D air cooled 4-stroke direct injection (DI) dieselengine was coupled to an electric motor and motored at speed and fuelconsumption conditions that simulated the G2 test modes from the ISO8178-4.2 [ISO 8178: Reciprocating internal combustion engines—Exhaustemissions measurement—Part 4. Test Cycles for different engineapplications. 1995, International Organization for Standardization].Only results from a load setting of 25% at 3600 RPM are presented here.The experimental system is shown schematically in FIG. 6. The fuelconsumption was measured by a gravimetric method using an Ohaus Explorerbalance, accurate to 0.1 g. The fuel injector 310 was removed from thecylinder head and placed into a spray chamber 320 with visual access tothe fuel spray 330. The chamber 320 was positioned so that the originalhigh pressure fuel line 340 could be used without modification oflength, although it was necessary to bend the fuel line.

The spray timing was monitored with a light attenuation method. AUniphase 0.95 mW Helium-Neon laser 350 was positioned so that a laserbeam 360 intersected the fuel spray at the injector orifice. During thespray event the laser was attenuated, changing the output voltage andenabling a clear transition at both the beginning and end of the spray.An AVL 364 shaft encoder mounted on the engine crank shaft enabled 0.1crank angle (CA) degree resolution of the spray event. A dataacquisition system 370 recorded the signals from the fuel line pressuresensor 380 and the phototransistor 390 located at the end of the laserbeam 360.

Fuel Samples

The tested fuel samples were a biodiesel fuel (B100), a methyl soyatefrom World Energy, Chelsea, Mass.; an unrefined soybean oil (soy oil)from Agricultural Commodities, Inc., New Oxford, Pa.; Norpar®-13(Norpar®), a normal paraffin mixture from C₁₁-C₁₅ from ExxonMobilChemicals, Houston, Tex.; a Fischer-Tropsch diesel (FT diesel) fromConocoPhillips Company, Houston, Tex.; a 15 ppm sulfur diesel fuel(BP-15) from British Petroleum—Fuels Technology, Naperville, Ill.; and abiodiesel/petroleum diesel blend (B20) consisting of methyl soyate fromWorld Energy and ultra low sulfur diesel fuel from British Petroleum.Table 1 comprises properties of these fuel samples.

TABLE 1 Properties of the fuel samples. Kinematic Density viscosity at15° C., at 40° C., Sulfur Boiling range* Flash Cetane Fuels g/cm³ mm²/s(ppm) (° C.) Point (° C.) number Norpar ® 0.762 2.36 <5 105 (IBP) 97 na(at 25° C.) 117 (DP) FT diesel 0.778 2.5 <1 205-212 (5%) 72.7-74.4 83325-339 (95%) 74.4 BP-15 0.837 2.5 15 203 (10%) 63.8 50 343 (95%) B200.846 2.7 13 198 (10%) 66.1 52.5 334 (95%) B100 0.888 4.1 <1 369 (5%)174 53 413 (95%) Soy Oil 0.91 31.3 na na >204 na na: not available *allboiling points were determined by the ASTM D-86 method, except for B100where the ASTM D2887 (SimDis) method was used.

The FT diesel was obtained by converting a synthesis gas stream with ahydrogen-to-carbon monoxide molar ratio of about 2:1 over aFischer-Tropsch cobalt-based catalyst in a slurry bubble reactor at atemperature of about 210-215° C. and a pressure of about 450 psig (about3200 kPa) so as to form a hydrocarbon synthesis product. The hydrocarbonsynthesis product was then hydrotreated over a nickel-based catalyst soas to substantially transform all of the olefins and oxygenates toparaffins; then fractionated in an atmospheric distillation column to atleast obtain a FT diesel fraction.

The experiments were performed in order to examine two separate issueswith regard to fuel formulation and engine emissions. The first was tostudy the difference in bulk modulus between biodiesel fuels and dieselfuels and the resulting effect on fuel injection timing. The second wasthe investigation of the potential impact of the use of paraffinicfuels, such as Fischer-Tropsch diesel fuels, on injection timing.

FIGS. 1 and 3 show results from the bulk modulus of compressibility fordiesel and biofuel blends, and the measurements of injection timing forbiofuel B100, baseline diesel, B20-FT blend and the normal paraffinsolvent (Norpar®). In FIG. 1, the bulk modulus of B20, B100 and the soyoil are higher than that of the baseline diesel fuel, consistent withthe results reported in the ‘Tat 2000 paper’. The bulk modulus of B20was slightly above that of the baseline diesel fuel BP-15. On the otherend, the bulk modulus of FT diesel, both FT diesel/biodiesel blends, andthe paraffinic solvent were lower than that of the baseline diesel fuel(BP-15). Blends of 20% and 40 vol. % of (methyl soy) biodiesel and a FTdiesel therefore displayed a lower bulk modulus of compressibility thanthe baseline diesel fuel, and should generate lower NOx emissions thatthe baseline diesel.

A relative spray intensity of 0.2 was used as an indication of thebeginning of light scattering by the fuel spray, providing a consistentmeans of quantifying the onset of the fuel spray. Accordingly, using 0.2relative spray intensity as an indication of the start of fuelinjection, FIG. 3 indicates that there was a 0.2 CA advance of fuelinjection timing for the petroleum diesel-biodiesel blend (B20), whilethere was an advance of 1.0 CA with pure biodiesel (B100). The purelyparaffinic solvent, Norpar®, retarded the fuel injection timing with thelargest retardation of 0.5 CA, while the B20-FT blend shows aretardation of 0.1 CA in fuel injection timing. Norpar® also showed thelowest bulk modulus of compressibility in FIG. 1. Since the B20-FT blendwith the synthetic FT diesel showed a retardation in fuel injectiontiming (−0.1 CA), the B20-FT blend is expected to generate less NOxemissions than the baseline fuel. On the other end, the B20 blend withthe ultra-low sulfur petroleum diesel showed an advanced injectiontiming (+0.2 CA), the B20 blend is expected to generate more NOxemissions than the baseline fuel.

In the present Example, the bulk moduli of blends of biodiesel and FTdiesel were examined to determine what blend ratio would be equivalentin bulk modulus to the baseline diesel (ultra-low sulfur diesel fuelBP-15) and what blend ratio would be have a bulk modulus below that ofthe baseline diesel. Table 2 shows the respective bulk moduliextrapolated from FIG. 1 at 5 MPa, 15 MPa and 25 MPa for paraffinicsolvent Norpar®, FT diesel, B20-FT, B40-FT, B20, B100 and soy oil.

TABLE 2 Bulk modulus of compressibility at 100° F. at differentpressures extrapolated from FIG. 1. B100 (methyl B20 with BP-15 PressureSoy Oil soyate) diesel diesel B40-FT60 B20-FT80 FT diesel Norpar ®  5MPa 1980 1650 1500 1460 1430 1370 1350 1240 15 MPa 2070 1755 1605 15801555 1495 1455 1350 25 MPa 2150 1870 1730 1690 1670 1615 1555 1460To illustrate the effect of biodiesel volume fractions on the bulkmoduli of the biodiesel/FT diesel blends, a plot of the bulk moduliextrapolated at 15 MPa (and measured at 100° F.) taken from Table 2 ofneat FT diesel, FT diesel/biodiesel blends and neat biodiesel B100versus the biodiesel volumetric fraction of the blends, is shown in FIG.2, wherein 0% represents neat FT diesel and 100% represents neat B100.From FIG. 2, one can visualize a blend of FT diesel and biodiesel whichwould lead to a “NOx neutral” formulation with a biodiesel volumefraction that would correspond to a bulk modulus of compressibility at15 MPa to be equal to about 1580 MPa, corresponding to the bulk modulusat 15 MPa for the baseline diesel. A B45-FT blend would correspond to abulk modulus of compressibility at 15 MPa of about 1580 MPa, equivalentto that of the baseline diesel. Hence the B45-FT blend comprised a “NOxneutral” biodiesel blend. Accordingly, the biodiesel-FT diesel blendswith less than 45% of biodiesel should generate a reduced level of NOxemissions when used in a diesel engine compared to conventional (crudederived) diesel formulations.

The data in these tests also showed a trend of increasing bulk modulusof compressibility with increasing density. As Table 3 shows, thedensity of the fuels considered here correlated directly with the bulkmodulus of compressibility.

TABLE 3 Fuel injection timing, bulk modulus of compressibility at 1000psi and 100° F., and specific gravity of various fuels. Crank Angle* forBulk Modulus at 1000 SOI Relative to psi and 100° F., Specific BP-15 MPaGravity Norpar ® −0.5 1262 0.762 FT diesel −0.5 1373 0.778 B20-FT −0.11390 nd BP-15 0 1477 0.837 B20** +0.2 1520 0.846 Soy oil 16 vol % +0.31579 0.852 B100** +1.0 1668 0.888 Soy oil nd 1996 0.91 *A positive (+)Crank Angle (CA) value represents an advance in injection timing,whereas a negative (−) CA value represents a retardation in injectiontiming. **B100 is methyl soyate and B20 comprises 20 vol % of B100.

The NOx emissions trends for the speciated biodiesel constituents andvarious biodiesel feedstocks shown in the ‘McCormick 2001 paper’ can beexplained, in light of the present work, on the basis of the variationof the bulk modulus of the fuels, consistent with the observations ofVan Gerpen and co-workers, such as are disclosed in the ‘Monyem paper’and in the ‘Tat 2000 paper’. These same observations have relevance tothe formulation of reformulated diesel fuels, FT diesel fuels, biodieselfuels (B20, B100, etc.) and blends thereof.

The present Example demonstrated that the higher bulk modulus ofcompressibility of biodiesel (specifically a methyl ester of soybeanoil) led to advanced injection timing. This advanced injection timinghas been shown in the literature to contribute to the well-documentedNOx emissions increase with the use of biodiesel fuel. An oppositetrend, a retarding of injection timing, was observed with paraffinicfuels because they have a lower bulk modulus of compressibility thanconventional diesel fuels. This supports the observation that paraffinicfuels such as Fischer-Tropsch diesel fuels yield lower NOx emissions.Thus, the observations of the biodiesel “NOx effect” reported in theliterature can be attributed to variations in the bulk modulus ofcompressibility of the fuel or fuel blend, and these effects correlatefor biofuels and paraffinic fuels quite well with fuel density.

The embodiments set forth herein are merely illustrative. Many varyingand different embodiments may be made within the scope of the presentinventive concept, including equivalent structures hereafter thought of,permutations, substitutions, or combinations of features from theembodiments herein detailed in accordance with the descriptiverequirements of the law. Many modifications may be made as well in theseembodiments. Because of these reasons, it is to be understood that thedetails herein are to be interpreted as illustrative and not in alimiting sense. Although the present invention and its advantages havebeen described in detail, it should be understood that various changes,substitutions and alterations may be made herein without departing fromthe spirit and scope of the invention as defined by the appended claims.

1-36. (canceled)
 37. A method of making a synthetic diesel fuel ordiesel fuel blendstock characterized by producing low sulfur and NOxemissions when used in compression ignition engines, said methodcomprising mixing a synthetic liquid distillate and a liquid biofuel ata ratio determined by equation:x _(b)=100(B−B _(sd))/(B _(b) −B _(sd)) wherein B represents a desiredbulk modulus of compressibility of the synthetic diesel fuel or dieselfuel blendstock at a specific pressure and a specific temperature;B_(sd) represents a measured bulk modulus of compressibility of thesynthetic distillate at the same pressure and temperature settings;B_(b) represents a measured bulk modulus of compressibility of theliquid biofuel at the same pressure and temperature settings; and x_(b)represents the calculated volume fraction of the liquid biofuel in theresulting mixture.
 38. The method of claim 37, wherein the desired bulkmodulus of compressibility B is between about 1300 MPa and about 1600MPa when measured at 15 MPa and 37° C.
 39. The method of claim 37,wherein the liquid biofuel comprises a compound selected from the groupconsisting of vegetable oils, alkyl esters of vegetable oils, mixturesof alkyl esters of fatty acids, and combinations thereof.
 40. The methodof claim 37, wherein the liquid biofuel comprises a mixture of alkylesters of fatty acids.
 41. The method of claim 37, wherein the liquidbiofuel comprises an alkyl ester of a vegetable oil selected from thegroup consisting of canola oil, soybean oil, cotton oil, palm oil,sunflower oil, and combinations thereof.
 42. The method of claim 37,wherein the liquid biofuel comprises an alkyl ester of a vegetable oilselected from the group consisting of canola oil, soybean oil, andcombinations thereof.
 43. The method of claim 37, wherein the liquidbiofuel comprises a total glycerin content less than 0.24 percent byweight of glycerin.
 44. The method of claim 37, wherein the liquidbiofuel has a flash point greater than 100° C.
 45. The method of claim37, wherein the liquid biofuel has an alcohol content less than 10percent by weight.
 46. The method of claim 37, wherein the liquidbiofuel has a kinematic viscosity at 40° C. between about 1.9 mm²/s andabout 6 mm²/S.
 47. The method of claim 37, wherein the synthetic liquiddistillate is derived from synthesis gas.
 48. The method of claim 37,wherein the synthetic liquid distillate is characterized by a boilingrange having a 5% boiling point between about 170° C. and about 210° C.and a 95% boiling point between about 320° C. and about 350° C.
 49. Themethod of claim 37, wherein the synthetic middle distillate ischaracterized by a sulfur content less than 10 ppm sulfur and by aspecific gravity less than about 0.8.
 50. The method of claim 37,wherein the synthetic middle distillate is characterized by a Brominenumber of less than 0.1 gBr/100 g.
 51. The method of claim 37, whereinthe synthetic middle distillate is characterized by an aromatics contentof less than about 1 percent by weight.